1. Field of the Invention
The present invention relates to calibrating a focus position of an optical module, and more particularly, to a circuit utilized a phase locked loop (DLL) for calibrating a focus position of an optical module and a calibrating method thereof.
2. Description of the Prior Art
A general camera module detects focus values (FV) corresponding to focus positions for deriving a mapping table of the focus positions and the focus values or a characteristic curve of the focus positions and the focus values when performing an auto focus operation (i.e. auto calibrating the focus position). Please refer to FIG. 1. FIG. 1 is a diagram illustrating a characteristic curve of the focus positions and the focus values and an optimal focus position derived using a mountain-climbing search method. As shown in FIG. 1, a solid line 110 represents the characteristic curve of the focus positions and the focus values, wherein the focus position corresponding to a biggest focus value is the optimal focus position (i.e. the peak position of the characteristic curve 110), and a dotted line is used to represent the moving direction and the moving step size of the focus position for searching the optimal focus position according to the mountain-climbing search method. When the camera module performs the focus operation, the mountain-climbing search method is a conventional method utilized by the camera module. As the operation of the mountain-climbing search method is well known to those skilled in the art, a detailed description is omitted here for the sake of brevity.
However, the characteristic curve 110 of the focus positions and the focus values should be derived before the mountain-climbing search method can be utilized to search for the optimal focus position. Ideally, the characteristic curve 110 is a symmetrical curve as shown in FIG. 1, and the characteristic curve 110 has only one peak position. However, due to noise interference, an actual characteristic curve sensed by the camera module is an asymmetrical curve as shown in FIG. 2a, where the characteristic curve shown in FIG. 2b has two or even more than two peak positions. Thus, the optimal focus position searched by the mountain-climbing search method may not be an actual optimal focus position. Therefore, since the camera module fails to take a photograph with the optimal focus position, the quality of the photographs or the images is significantly degraded.